1. Technical Field
The present invention relates to a charging device for charging a secondary battery such a lithium ion secondary battery, and more particularly to a charging device having a protecting switching element for interrupting an unnecessary discharging path formed to a charging secondary battery connected to an output line of the charging device.
2. Background Art
As a power source for driving an electric cordless tool, a secondary battery whose capacity is relatively increased such as a nickel hydrogen battery or a nickel cadmium battery is used. As a high capacity and light secondary battery, a lithium ion battery has been put to practical use. The lithium ion battery has features that a nominal voltage of the lithium ion battery is relatively higher than that of the nickel hydrogen battery or the nickel cadmium battery and compact and light. Further, the lithium ion battery has features that the lithium ion battery is good in its discharging efficiency, can discharge even in an environment of relatively low temperature and can obtain a stable voltage within a wide temperature range.
On the other hand, in a charging device for charging the secondary battery (a battery pack) such as the nickel hydrogen battery or the lithium ion battery, a device is formed for preventing an unnecessary discharge or charge to ensure the life of the battery. Especially, in the lithium ion battery, since to carry out an over-discharging operation or an over-charging operation may possibly give a damage to the battery, as disclosed in JP-A-6-141479, a protective countermeasure is ordinarily carried out that an exclusive protecting IC or a microcomputer is ordinarily provided in a battery pack to monitor an over-charge or an over-discharge, and when a battery voltage is a prescribed voltage value or lower, or higher, the protecting IC or the microcomputer outputs a control signal to interrupt a charging path or a discharging path of the secondary battery in accordance with the control signal.
Further, for instances as disclosed in JP-A-2004-187366, a charging device is known that a relay switch is inserted into an output line of the charging device and when a charging operation is completed, the relay switch is interrupted to electrically disconnect the secondary battery.